An enzyme, endoR.HhaII, will be studied to determine how a protein molecule can recognize a particular sequence in DNA, the chemical which controls heredity. Since this enzyme cleaves DNA (but only after recognizing a short, specific sequence in the DNA), it should serve as a good model for this kind of investigation. By obtaining altered forms of the enzyme which can no longer recognize the proper sequence in DNA, one can ascertain those places in the structure of this enzyme which are directly involved in the recognition process. This information should be useful in understanding how other sequence-specific proteins interact with DNA, especially those controlling the action of genes.